This invention relates to a catalytic hydrocracking process and a regenerable catalyst for use therein. More particularly, the invention relates to a hydrocracking catalyst, and most especially to noble metal, zeolitic hydrocracking catalysts, having highly improved regeneration properties.
Petroleum refiners often produce desirable products, such as gasoline and turbine fuel, by catalytically hydrocracking high boiling hydrocarbons into product hydrocarbons of lower average molecular weight and boiling point. Hydrocracking is generally accomplished by contacting, in an appropriate reactor vessel, a gas oil or other hydrocarbon feedstock with a suitable hydrocracking catalyst under appropriate conditions, including an elevated temperature and an elevated pressure and the presence of hydrogen, such that a hydrocarbon product is obtained containing a substantial proportion of a desired product boiling in a specified range, as for example, a gasoline boiling in the range of 185.degree. to 420.degree. F.
Oftentimes, hydrocracking is performed in conjunction with hydrotreating, usually by a method referred to as "integral operation." In this process, the hydrocarbon feedstock, usually a gas oil containing a substantial proportion of components boiling above a desired end point, as for example, 420.degree. F. in the case of certain gasolines, is introduced into a catalytic hydrotreating zone wherein, in the presence of a suitable catalyst, such as a non-zeolitic, particulate catalyst comprising a Group VIII metal component and a Group VIB metal component on a porous refractory oxide support most often composed of alumina, and under suitable conditions, including an elevated temperature (e.g., 400.degree. to 1000.degree. F.) and an elevated pressure e.g., 100 to 5000 p.s.i.a.) and with hydrogen as a reactant, the organonitrogen components and the organosulfur components contained in the feedstock are converted to ammonia and hydrogen sulfide, respectively. Subsequently, the entire effluent removed from the hydrotreating zone is treated in a hydrocracking zone maintained under suitable conditions of elevated temperature, pressure, and hydrogen partial pressure, and containing a suitable hydrocracking catalyst, such that a substantial conversion of high boiling feed components to product components boiling below the desired end point is obtained. Usually, the hydrotreating and hydrocracking zones in integral operation are maintained in separate reactor vessels, but, on occasion, it may be advantageous to employ a single, downflow reactor vessel containing an upper bed of hydrotreating catalyst particles and a lower bed of hydrocracking particles. Examples of integral operation may be found in U.S. Pat. Nos. 3,132,087, 3,159,564, 3,655,551, and 4,040,944, all of which are herein incorporated by reference. In some integral operation refining processes, and especially those designed to produce gasoline from the heavier gas oils, a relatively high proportion of the product hydrocarbons obtained from integral operation will have a boiling point above the desired end point. For example, in the production of a gasoline product boiling in the C.sub.4 to 420.degree. F. range from a gas oil boiling entirely above 570.degree. F., it may often be the case that as much 30 to 60% by volume of the products obtained from integral operation boil above 420.degree. F. to convert these high boiling components to hydrocarbon components boiling below 420.degree. F., the petroleum refiner separates the 420.degree. F.+high boiling components from the other products obtained in integral operation, usually after first removing ammonia by a water washing operation, a hydrogen-containing recycle gas by high pressure separation, and an H.sub.2 S-containing, C.sub.1 to C.sub.3 low BTU gas by low pressure separation. The resultant denitrogenated and desulfurized liquid is then distilled into a C.sub.4 to 420.degree. F. overhead gasoline product and a 420.degree. F.+unconverted fraction. This bottom fraction is then subjected to further hydrocracking in a second hydrocracking zone wherein yet more conversion to the desired C.sub.4 to 420.degree. F. product takes place.
In the foregoing process, the two hydrocracking reaction zones often contain hydrocracking catalysts of the same composition. One catalyst suitable for such use is disclosed as Catalyst A in Example 16 of U.S. Pat. No. 3,897,327, herein incorporated by reference in its entirety, which catalyst is comprised of a palladium-exchanged stabilized Y zeolite hydrocracking component. But although the catalysts used in the two hydrocracking reaction zones may have the same composition and the same catalytic properties, the hydrocracking conditions required in the second hydrocracking reaction zone are less severe than those required in the first. The reason for this is that ammonia is not present in the second hydrocracking reaction zone (due to water washing) whereas a significant amount of ammonia is present in the first hydrocracking zone. To account for the difference in operating conditions, it is believed that ammonia neutralizes or otherwise interferes with the acidity of the zeolite in the catalyst of the first reaction zone, thereby forcing the refiner to employ relatively severe conditions for operation, as for example, increased temperature. On the other hand, in the ammonia-deficient atmosphere of the second hydrocracking reaction zone, high conversions to the desired product are obtainable under relatively moderate conditions, often with an operating temperature about 150.degree. to 210.degree. F. lower than that required in the first hydrocracking reaction zone.
It has been discovered, however, that a difficult problem presents itself when the hydrocracking catalyst in the second hydrocracking zone must be regenerated. During hydrocracking, coke materials deposit on the catalyst particles, and since the coke obviously interferes with the activity of the catalyst, it is necessary to periodically regenerate the catalyst by combustion of the coke. Curiously, it has been found that, after regeneration, the catalyst used in the second hydrocracking reaction zone loses substantial activity for hydrocracking under the relatively moderate conditions employed therein. Even more curiously, it has been found that, assuming identical catalysts are used in the two hydrocracking zones, both remain useful after regeneration for use in the first reaction zone, but both exhibit substantial activity losses compared to fresh catalyst if used in the second hydrocracking reaction zone.
Many attempts have been made to overcome the detrimental effects associated with regenerating hydrocracking catalysts for use in the ammonia-deficient environments of the second hydrocracking zone, and particularly with respect to catalysts containing noble metal-exchanged zeolites. But these attempts have largely focused on methods for restoring some or all of the catalytic activity lost through regeneration or other high temperature operation. These restoration (or rejuvenation) methods generally involve treating the regenerated catalyst or the coked catalyst prior to regeneration with an ammonium salt, ammonium hydroxide, gaseous ammonia, or mixtures thereof. Descriptions of typical prior art rejuvenation methods employing ammoniated media may be found in U.S. Pat. Nos. 3,692,692, 3,835,028, 3,849,293, 3,899,441, 3,943,051, 4,002,575, 4,107,031, 4,139,433, and 4,190,553. The general theory behind these methods is that the activity losses of catalysts used in hydrocracking environments are caused by the agglomeration of the otherwise dispersed Group VIII active metal hydrogenation component, and the ammoniation treatments of the prior art aim to reverse this mechanism and redisperse the Group VIII active metal component.
Although the prior art methods have met with some success, a major difficulty in their use is that the rejuvenation of the catalyst must be performed under carefully controlled conditions in the presence of ammonia or an ammonium ion-containing medium, with all the attendant equipment and chemical costs associated therewith. Further, and far more importantly, by focusing on rejuvenation procedures, the prior art has aimed at correcting a problem (catalyst deactivation) once it has come into existence rather than preventing the problem by providing a catalyst resistant to deactivation during regeneration. Further still, the prior art procedures are of only limited usefulness, being applicable, for example, to catalysts containing palladium-exchanged zeolites stabilized with magnesium cations but being of at most only limited usefulness with many other hydrocracking catalysts. As an illustration, hydrocracking catalysts containing hydrogen-palladium zeolites have been found to be highly sensitive to ammonia or ammonium ion treatments, with collapse of the zeolitic crystal structure and virtually complete loss of catalytic activity being the end result of such treatment.
Accordingly, it is a major object of the invention to provide a zeolitic hydrocracking catalyst resistant to deactivation during regeneration and other high temperature operations. It is a further object to provide a Group VIII metal-exchanged zeolite, and particularly a noble metal-exchanged zeolite, and a method for preparing such a zeolite, which is useful in a hydrocracking catalyst and resistant to losses in catalytic activity under high temperature conditions, particularly the high temperature oxidizing conditions that pertain during regeneration of hydrocracking catalysts. Yet another object of the invention is to provide a noble metal-exchanged, zeolitic hydrocracking catalyst, and a method for preparing such a catalyst, for use in a catalytic hydrocracking process wherein high temperature regenerations of the catalyst are periodically required without incurring substantial losses in catalytic hydrocracking activity. A further object is to provide a hydrocracking process taking advantage of the regeneration-resistant properties of the zeolite and hydrocracking catalyst of the invention. These and other objects of the invention will become more clear in light of the following description of the invention.